1. Field of the Invention
The present invention relates to a thin film transistor (TFT) of the type used in a liquid crystal display (LCD). More particularly, it relates to a polycrystalline silicon thin film transistor (Poly-Si TFT) and a method of manufacturing the same.
2. Discussion of the Related Art
Generally, in order to form a polycrystalline silicon membrane, intrinsic amorphous silicon is first deposited by conventional methods, for example, Plasma Chemical Vapor Deposition (PCVD) or Low Pressure CVD (LPCVD). Then that amorphous silicon is crystallized. The crystallization methods can be classified into three types.
Firstly, a metal induced crystallization (MIC) technique. The MIC technique can use large size glass substrates, since the MIC technique can use a low crystallization temperature.
Secondly, a solid phase crystallization (SPC) technique. SPC changes amorphous silicon into polycrystalline silicon by heat-treatment at a high temperature for a long time. It requires forming a buffer layer on a quartz substrate in order to prevent the quartz substrate from diffusing an impurity material. The amorphous silicon layer is then deposited on the buffer layer and crystallized by the heat-treatment.
Thirdly, a laser annealing technique grows polycrystalline silicon using laser irradiation while heating a substrate having an amorphous silicon membrane.
Although metal induced crystallization (MIC) can form polycrystalline silicon on large sized glass substrates, the quality of the resulting membrane is questionable due to the high possibility of residual metallic material in the grain boundaries of the polycrystalline silicon.
The solid phase crystallization (SPC) method results in irregular grain boundaries that cause the gate insulating layer on the polycrystalline silicon layer to grow erratically, leading to a low breakdown voltage. Furthermore, since the grain sizes of the polycrystalline silicon are highly non-uniform, the electrical properties of the resulting device, such as the on-current and the threshold voltage, are not good. Finally, a costly quartz substrate should be employed when performing SPC.
The laser annealing technique is widely used and has been carefully investigated. This method can heat amorphous silicon on a glass substrate up to the melting temperature of the silicon without damaging the substrate.
For a more complete understanding of laser annealing, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1a to 1d show the fabrication process steps of a TFT according to the related art. Referring to FIG. 1a, a first insulating layer 2 and an amorphous silicon layer 4 are sequentially formed on a substrate 1. The first insulating layer 2 prevents the substrate 1 from effusing alkali. The first insulating layer 2 also acts as an antireflection layer the prevents leakage of the excimer-laser beam used in laser annealing.
FIG. 1b shows the process step of crystallizing the amorphous silicon layer 4. An excimer-laser beam quickly heats the amorphous silicon to its melting temperature. If the excimer-laser irradiation stops, the amorphous silicon cools down quickly. Crystal growth of polycrystalline silicon then occurs with the numerous crystal nuclei acting as starting points or “seeds.”
Referring to FIG. 1c, an island-shaped polycrystalline silicon 4 layer is formed by patterning the crystallized poly-silicon. A second insulating material and a first metallic material are deposited sequentially and patterned to form a second insulating layer 6 and a gate electrode 8. Then, the peripheral portion of the polycrystalline silicon 4 is ion-doped using the gate electrode 8 as a mask.
As shown in FIG. 1d, a third insulating layer 11 is deposited and patterned to form contact holes on the doped portions 12 and 13. Then, source and drain electrodes 9 and 10 are formed in and around the contact holes.
However, the conventional laser annealing method uses the numerous crystal nuclei as starting points (seeds) at random. The grain size of the polycrystalline silicon is then about 1 μm. From these results, as shown in FIG. 2, many small-sized grains 20 are formed in the channel region of the TFT. This means that the grain boundaries, which affect the electrical properties, are numerous. This deteriorates the electrical properties of the TFT.
Referring to FIG. 3, the excimer-laser beam should be irradiated many times to obtain a reliable TFT when large-sized amorphous silicon on a substrate 40 is being crystallized. To accomplish this a second excimer-laser beam 44 can be irradiated over about 90% of the area irradiated by the first excimer-laser. The amorphous silicon is then converted to polycrystalline silicon by repeating the overlap of the laser beams several times.
As the conventional method forms small-sized grains and numerous grain boundaries, it does not produce good electrical properties when the length of TFT channel region is around 10 μm.
To address these problems, Japanese Journal of Physics (JJAP), pp. 4545-4549, in 1992 and JJAP, pp. 70-74, in 1994 disclose TFTs having a gate electrode with a bridge structure. FIGS. 4 and 5 explain those techniques.
An insulating layer 52 is formed on a silicon wafer 50, which has been constructed by removal of the central portion 50b of the silicon Wafer 50 using preferential etching. Then, amorphous silicon 54 is formed on the insulating layer 52. That silicon is then crystallized using an excimer-laser. The grain size of the polycrystalline silicon grows up to 50 μm. In this method, the peripheral portion 50a of the silicon wafer 50 acts as the bridge, which supports the central portion 50b of the silicon wafer 50.
FIG. 5 shows a cross sectional view of a TFT that is made of polycrystalline silicon formed using that crystallization method (in FIG. 4). A gate electrode 56 is formed under the silicon wafer 52. Patterning yields the polycrystalline silicon active layer 54. After that, source and drain electrodes 58 and 60 are formed.
The field-effect mobility of a TFT fabricated by the method of FIGS. 4 and 5 is similar to that of a MOS (metal oxide silicon) transistor made of amorphous silicon.
These techniques, however, do not explain the method of forming the preferable grain size and shape. Furthermore, the method of fabricating plural uniform devices on a large size substrate is not suggested.